1. Field of the Invention
The present invention relates to a nuclear fuel assembly for use in nuclear reactors, and in particular, to an apparatus for operating the hold down springs, grid springs, provided in the grids of the nuclear fuel assembly for holding the fuel rods firmly therein.
2. Background Art
Fuel rod assemblies such as the one disclosed in U.S. Pat. No. 5,068,081 shown in FIG. 6 have been known.
In this figure, the numerals 1 and 2 refer to a top nozzle and a bottom nozzles, respectively, which are disposed vertically and oppositely spaced apart, and between the top nozzle 1 and the bottom nozzle 2 are a plurality of rigidly fixed control-rod guide pipes 3 (hereinbelow referred to as guide pipes 3), and in the mid section of the guide pipes 3 are a plurality of grids 4 disposed vertically and spaced apart from each other.
The grids 4 are, as shown in FIGS. 7 to 11, constructed of a plurality of outer straps 7a (the straps placed on the outer sides of the grids 4) made of thin metal strips, and inner straps 7b (all other straps) having slits 8 spaced at regular intervals in the longitudinal direction. By inserting the slits 8 into each other's slits 8, a plurality of grid cell spaces 5 (hereinbelow referred to as grid cells 5) are formed. Each wall of the grid cells 5 is provided with a dimple 9, and outer springs 10a (which are formed on the outer straps 7a), or inner springs 10b (which are formed on the inner straps 7b). A fuel rod 6 inserted into a grid cell 5 is pressed against the dimple 9 by either the outer spring 10a or by the inner spring 10b, thereby supporting the fuel rod 6 firmly in the grid cell 5 with the springs.
The outer strap 7a has a pair of slits 11 which pass through both sides of the outer spring 10a.
The conventional method of assembling such a fuel assembly will be described.
First, the grids 4 are placed apart with a certain distance. The guide pipes 3 are inserted into the specified oppositely paired grid cells 5, and are then rigidly attached to the grids 4. Subsequently, the fuel rods 6 are inserted into the space of the grid cells 5 by sliding the rods 6 against the outer and inner springs 10a-10b and dimples 9. The rods 6 are thereby secured in the grid cells 5 via the pressing force exerted by the springs 10a-10b against the dimples 9. When all of the rods 6 are inserted into the grid cells 5, the top and bottom nozzles 1 and 2 are firmly attached to the opposite ends of the respective guide pipes 3.
In the conventional assembling method described above, when the rods 6 are being inserted, by sliding through the dimples 9 and springs 10a-10b, surface damage (fine scratches) were sometimes introduced on the surface of the fuel rods 6.
Therefore, there has been a technique to prevent the formation of such scratches, with the use of an inner key 12, shown in FIG. 11, which is inserted into a grid cell 5, so as to retract the inner spring 10b away from the grid cell 5, before inserting the fuel rod 6. Similarly for the outer springs 10a, the use of such keys has been examined.
In the conventional method, however, the operations involving the key insertion have been performed manually, and the entire operation has been troubled by the process inefficiency due to cumbersome and lengthy steps. Further, the care of the key quantity was difficult to take, because of the large number of keys involved.